In many cases the population of ions transmitted to an ion mobility spectrometer or separator contains ion species which are either not of interest to a particular analysis or are in such high abundance that they can cause either saturation of a downstream ion detector or distort the drift time or peak shape recorded in ion mobility separations due to space charge interaction.
The presence of such ion species within an analytical ion mobility spectrometer or separator (“IMS”) device can lead to non-optimum performance of the ion mobility spectrometer or separator.
Distortions due to space charge interaction can occur for high charge densities within specific ion mobility ranges since ions within these regions may experience high charge density for most or all of their transit time through the ion mobility spectrometer or separator.
U.S. Pat. No. 7,105,808 (Bromberg) discloses an arrangement comprising an atmospheric pressure drift tube with an ion shutter grid and an upstream pre-concentrator. The pre-concentrator consists of a series of at least two grids. Ions of high mobility are selectively removed as they travel further into the pre-concentrator region and are neutralised on the surface of a grid. Ions pass through several grids resulting in more transmission losses.
Ions are gated into the pre-concentrator region for a very short period of time (e.g. about 100-200 ms) and are then oscillated back and forth between two grids to allow depletion of the high mobility species. The remaining ions are then gated into the ion mobility drift tube and separated. Most of the ions created in the ion source are lost as no ions are accumulated prior to the pre-concentrator.
The small portion of the ion population gated into the pre-concentrator region may not be allowed to reside for an indefinite period as radial diffusion of the ion cloud will either exceed the radius of the drift tube during pre-concentration or subsequently during ion mobility separation and hence ions will be lost to the walls of the drift tube.
It should be noted that pre-concentration in this case refers to the enriching of the ion population with respect to low mobility ions.
According to the arrangement disclosed in U.S. Pat. No. 7,105,808 (Bromberg) all ions above a certain mobility are lost to the system before the remaining ions are transferred to the ion mobility drift tube. It is not therefore possible to control the amount of high mobility ions that are transferred.
Furthermore, the system disclosed in U.S. Pat. No. 7,105,808 (Bromberg) suffers from a low duty cycle since the arrangement of the grids and pre-concentrator causes numerous transmission losses.
Attenuation devices are known. Reference is made, for example, to the arrangements disclosed in U.S. Pat. No. 7,683,314 (Micromass).
US 2002/0014586 (Clemmer) discloses an instrument for separating ions in time as functions of preselected ion mobility and ion mass.
U.S. Pat. No. 6,498,342 (Clemmer) discloses an ion separation instrument.
GB-2502650 (Micromass) discloses an adaptive and targeted control of ion populations to improve the effective dynamic range of a mass analyser.
US 2011/0183431 (Covey) discloses a mass analysis system with low pressure differential mobility spectrometer.
“Atmospheric pressure ion trapping in a tandem FAIMS-FAIMS coupled to a TOFMS: Studies with electrospray generated gramicidin S ions” (Guevremont et al.), J Am Soc Mass Spectrom. 2001 December; 12(12): 1320-30 discloses a tandem FAIMS-FAIMS system for ion trapping at room temperature and atmospheric pressure.
US 2012/119078 (Micromass) discloses an ion population control device for a mass spectrometer.
It is desired to provide an improved method of mass spectrometry.